When wildfires burn, they release particulate matter, or PM2.5—ultra-fine particles that fill the air with pollution and can penetrate our lungs and bloodstream. But when fires burn through urban areas, as they have been doing across Los Angeles, it’s not only vegetation contributing to that pollution. Entire homes—and all the plastics, metals, and electronics inside them—are going up in flames, resulting in a “toxic soup” of smoke and ash that’s even more dangerous to our health. And those hazardous particles could linger inside homes for much longer than the fires themselves.
Studies about previous wildfires that stretch into urban areas—fires in the wildland-urban interface, as they’re called—found that air quality inside nearby homes was affected for weeks after a blaze. People also reported health symptoms that they attributed to that poor air quality for months after a fire.
Joost de Gouw, a chemistry professor and fellow with the Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder, studied this effect after the Marshall Fire in Colorado in 2021. That blaze, fueled by 100-mile-per-hour winds and record dry conditions in winter—similar to the environment that’s fueling the L.A. fires—burned more than 1,000 buildings. More than 37,000 people evacuated.
“A lot of the people who were evacuated and returned to unburned homes were initially, of course, very happy,” de Gouw says. “But then that was replaced pretty quickly by concern about the environmental impact on their homes.” Many residents found a lot of ash and soot inside their houses, and also noticed a campfire or chemical fire odor.
De Gouw’s team began monitoring the situation, installing air quality instruments about 10 days after the Marshall Fire in homes on the fire’s perimeter. They found that concentrations of volatile organic compounds inside those Colorado homes were similar to the levels of pollution in downtown Los Angeles in the 1990s (L.A.’s air quality has gotten noticeably cleaner since then.) That pollution lasted inside the homes for about five weeks.
It was surprising to de Gouw how long that smoke lingered. “Before any of this happened, I would have predicted it would take hours or days” for the indoor air quality to go back to normal, he says. “In reality, it took weeks. . . . What it means is that your home can soak up a lot more pollutants—that are then later returned to the air—than we would have imagined. And we don’t know what materials of your home do this.”
Researchers also sent surveys to Colorado residents both within the burn perimeter and a random sample of those within two miles of the fire, asking them about the conditions of their home and their health. Six months after the fire, 55% of respondents reported symptoms like headaches or sore throats that they contributed to bad air quality. Residents who found ash inside were three times as likely to report headaches compared to those respondents who didn’t find any ash. Those who reported an odor in their home were four times as likely to report headaches compared to residents who didn’t notice an usual scent after the fire.
The L.A. fires are still burning, and the priority—as it should be—is now containing the fires and preventing further damage and loss of lives. “But once the fires are out, the focus will, in our experience, change to clean up. That’s when these issues will come to the foreground,” de Gouw says.
The scale of the L.A. fires makes this an even greater concern. In Colorado’s Marshall Fire, just over 1,000 buildings burned. In L.A., already more than 10,000 structures have burned, making the Palisades and Eaton Fires the most destructive in L.A. County history. That impacts overall pollutant exposure. “If you’re downwind from 1,000 burning homes versus 10,000 burning homes, that makes a difference,” de Gouw says.
The perimeter of the L.A. fires will be greater than the Marshall Fire, which burned just over 6,000 acres versus the more than 40,000 in L.A. California residents have been reporting ash and other debris in the air outside of mandatory evacuation zones. Particulate matter can also travel far; the L.A. fires are impacting air quality in San Diego County, though the winds have blown some of the smoke toward the ocean.
California homes are also older than those in Colorado, which creates different concerns. In Colorado, most of the burned structures were from the 1990s or 2000s; in L.A., buildings can date back to the ‘30s, ‘40s, and ’50s. “Older homes have different materials in them,” he notes. If a structure that had used lead paint burns, for example, then that lead gets into the air.
It’s hard to say exactly what will happen after the L.A. fires, but if people return and find that their homes withstood the flames, they may then have to be concerned about the ash and smoke that got inside—and will linger in—their houses. Though the effects of wildfire smoke have been studied, scientists don’t yet know much about the effects of smoke from so many burning buildings and cars.
When the cleanup process begins, de Gouw recommends wet cleaning surfaces to get rid of any ash, and using vacuum cleaners with HEPA filters. Research has shown that the cleanup process can also “re-suspend” ash into the air, so he adds that it’s good to wear a mask for protection. For smoke odor, air purifiers with activated carbon filters can be effective (and can be made from items at a hardware store). Also opening windows and increasing ventilation works, “but of course you can only do that when the outside air is back to normal,” he notes.
